Most of tritium existing on the ground is in the form of tritium water as an oxide, i.e., tritiated water. The concentration of tritiated water circulating in the atmosphere is considered to have roughly a fixed value in all ages and countries and including animals and plants. A period during which tritiated water falls out of atmospheric circulation can be detected from an amount of decrease in underwater concentration, so that the age of groundwater can be measured. This measurement is utilized for practical examination on flow of groundwater in the fields of civil engineering and agriculture. Tritium exists in water in the form of tritiated water with tritium bound with oxygen. The tritiated water is dispersively distributed widely in vapor, rainfall, groundwater, river water, lake water, sea water, drinking water and organisms as a gas phase, a liquid phase and a solid phase in the hydrosphere.
Natural tritium is generated by a reaction of a cosmic ray with the air, and the amount of natural tritium is very small because its generation probability is very small. On the other hand, tritium generated through nuclear tests in the 1950s, nuclear reactors and reprocessing of nuclear fuels is released in the environment in a large amount (fall-out tritium). In an installation associated with a nuclear reactor, tritium is generated during operation and maintenance of the reactor and reprocessing of nuclear fuels, and accumulated and localized at a higher level in comparison with the external environment. The tritium is systematically released to the atmosphere and sea because its chemical nature is almost comparable to that of hydrogen.
The highest value measured in Japan is 1,100 Bq/L, a value detected in Jun. 21, 2013 at a dedicated port within the site of Fukushima No. 1 Nuclear Plant where the nuclear accident occurred. Since tritium is difficult to chemically separate from hydrogen, studies on a method for physically separating tritium have been conducted. However, remained only at an experimental level, and such a method has not been put into practical use yet. Therefore, radioactivity from tritium released in the environment due a nuclear accident etc. cannot be eliminated by current techniques. The amount of contaminated water containing tritium generated in Fukushima No. 1 Nuclear Plant is said to reach an 800,000 m3 scale in future, and early establishment of an effective method for disposing of the contaminated water is desired.
As methods for collecting tritium, a water distillation method using a vapor pressure of H2O, HTO and T2O, a water-hydrogen exchange method using an exchange reaction of H and T atoms, a double temperature exchange method using an isotope chemical equilibrium shift, and a water electrolytic method using a gas generation potential difference are conceivable. However, efficient separation of tritium is difficult in the methods except for the water electrolytic method.
On the other hand, in the electrolytic method, the separation factor is considerably high, so that tritium can be efficiently separated.
However, the tritium concentration is extremely low, and therefore when the concentration of tritium is measured, electrolytic concentration is generally performed for improvement of measurement accuracy. Here, for electrolytic concentration of heavy water, a method has been heretofore known in which a sample solution with an electrolyte dissolved therein is prepared, and electrolysis is performed with tabular flat shapes faced each other. Water contained in an electrolytic solution includes HDO and HTO in addition to H2O, and they are usually decomposed into hydrogen and oxygen according to water decomposition. The isotope effect causes decomposition of H2O to precede decomposition of HDO and HTO, so that the concentrations of deuterium and tritium are increased to perform concentration. In the electrolytic concentration, nickel is used as an anode, and steel, iron, nickel and the like are used as a cathode. These electrodes are cleaned, a water sample prepared by adding lean caustic soda as a support salt to a solution of water including heavy water is added in a glass container, and electricity is supplied to perform electrolysis. At this time, with the current density set to about 1 to 10 A/dm2, concentration of heavy hydrogen is performed usually until the liquid amount decreases by a factor of 10 or more while the liquid temperature is kept at 5° C. or lower for preventing evaporation of water by generation of heat.
That is, electrolytic concentration of tritium takes advantage of the disposition in which tritiated water is harder to be electrolyzed than light hydrogen water as in the case of the heavy water. The method including inserting metal electrodes in an alkaline aqueous solution to perform electrolysis has been already subjected to many studies, and publicly formalized as a standard method. In this method, tritium concentration is performed in a single stage. In practice, however, conventional electrolytic concentration methods have some problems. These problems include the following: experimental operations are complicated; the tritium concentration ratio is limited by the upper limit of the electrolyte concentration; a mixed gas of hydrogen and oxygen may be generated to cause explosion; much time is required for electrolysis; and electric power consumption becomes enormous, so that it is difficult to treat a large volume of water.
The present inventors developed a method for electrolytically concentrating heavy water, which solves the problems of conventional techniques, and is capable of electrically concentrating and fractionating a raw water containing a large amount of heavy water by an alkaline water electrolytic method, and also producing a high-purity hydrogen gas and/or a high-purity oxygen gas, as a method for treating raw water containing a large amount of tritiated water by alkaline water electrolysis, and applied for a patent thereof (Patent Literature 1).
According to Patent Literature 1, there can be provided a method for electrolytically concentrating heavy water, the method including electrolytically concentrating heavy water using an alkaline water electrolytic bath including: an anode chamber for storing an anode; a cathode chamber for storing a cathode; and a diaphragm for dividing the anode chamber and the cathode chamber from each other, wherein from a circulation tank for storing an electrolytic solution with high-concentration alkaline water added to raw water including heavy water containing tritium, the electrolytic solution is circulated and fed to both electrolytic chambers; the anode chamber to which an anode-side gas-liquid separation device and anode-side water sealing device are connected; the cathode chamber to which a cathode-side gas-liquid separation device and cathode-side water sealing device are connected; electrolysis is continued to concentrate heavy water in the electrolytic solution while the electrolytic solution from which a generated gas is removed by the anode-side gas-liquid separation device and the cathode-side gas-liquid separation device is circulated and fed to the circulation tank to keep constant the alkali concentration of the electrolytic solution fed into both the electrolytic chambers; a hydrogen gas is collected or discarded by the cathode-side gas-liquid separation device, and an oxygen gas is collected or discarded by the anode-side gas-liquid separation device.
Further, according to the method described in Patent Literature 1, a radioactive waste containing a large amount of tritium can be efficiently concentrated and fractionated by electrolysis, and a high-concentration and high-purity hydrogen gas and oxygen gas can be efficiently collected.
However, the method described in Patent Literature 1 has the disadvantage that electric power consumption becomes enormous as described above, and this disadvantage is the biggest obstacle to employment of an electrolytic method.
The present inventors conducted studies on reduction of electric power consumption by combination of an alkaline water electrolytic method and a fuel cell as a method for solving the disadvantage.
That is, in the water electrolytic method, a hydrogen gas and an oxygen gas are generated. Heretofore, these gases have been discarded, but a hydrogen gas and an oxygen gas which have been discarded heretofore can be used as a raw material in a fuel cell. Accordingly, the present inventors conducted studies on a method using, as a power source in a water electrolytic method, a fuel cell using as raw materials a hydrogen gas and an oxygen gas generated in the water electrolytic method.
Fuel cells are classified into the following types based on the type of electrochemical reaction and electrolyte.
(1) Alkaline fuel cell (AFC)
(2) Phosphoric acid fuel cell (PAFC)
(3) Molten carbonate fuel cell (MCFC)
(4) Solid oxide fuel cell (SOFC)
(5) Proton conductive fuel cell (PEFC)
(6) Direct methanol fuel cell (DMFC)
(7) Bio-fuel cell (MFC)
(8) Direct formic acid fuel cell (DFAFC)
The currently mainstream fuel cells are proton conductive fuel cells (PEFC). The proton conductive fuel cell exhibits sufficient electric power generation performance with a hydrogen fuel. However, the proton conductive fuel cell has the problem of a high cost and small resource amount associated with noble metals because the fuel cell operates under a strong acid atmosphere, and therefore the catalyst to be used is almost limited to a platinum-based noble metal.
The PEFC includes a fuel electrode, an oxygen electrode and an electrolyte layer. A solid polymer (cation exchange membrane) containing a strong-acidic electrolyte aqueous solution is used for the electrolyte layer. A hydrogen gas is introduced into the fuel electrode, an oxygen gas is introduced into the oxygen electrode, the following reactions take place at the electrodes, and as a whole, water is generated according to the following reaction.
Whole 2H2+O2→2H2O
Fuel electrode (negative electrode) H2→2H++2e−
Oxygen electrode (positive electrode) 4H++O2+4e−→2H2O
Protons (H+) generated at the fuel electrode diffuse through the solid polymer membrane (cation exchange membrane) to move to the oxygen electrode side, and react with oxygen (O2) to generate a mist (H2O), which is discharged from the oxygen electrode side.
Meanwhile, as the fuel cell, an alkaline electrolyte type fuel cell (AFC: alkaline fuel cell) is known. In the alkaline electrolyte type fuel cell, hydroxide ions are used as an ion conductor, and a separator between electrodes is impregnated with an alkaline electrolytic solution to form a cell. Like the PEFC, a type of cell using a polymer membrane has also been reported. The AFC is a fuel cell which has high reliability and is practically used in aerospace applications etc. because the AFC has the simplest structure and is used in an alkali atmosphere, and therefore an inexpensive electrode catalyst such as a nickel oxide can be used, and because a liquid electrolyte is used at normal temperature, and therefore the cell configuration can be simplified.
Meanwhile, in the case where hydrogen is extracted from a reformed hydrocarbon-based fuel, the alkaline electrolytic solution generates a carbonate to be degraded if a hydrocarbon is mixed therein. Similarly, when air is used as an oxidant, the electrolytic solution absorbs carbon dioxide to be degraded, and therefore it is necessary to use high-purity oxygen as an oxidant. For improving the purity of hydrogen, the fuel is made to pass through a palladium membrane to improve the purity of hydrogen. Since the electrolyte is an aqueous solution, the operation temperature range is limited to temperatures at which the electrolytic solution is not frozen and evaporated. Since the mobility (diffusion coefficient) of ions varies depending on the temperature, so that electric power generation is affected, the temperature condition is severe. Since the activity of a nickel-based catalyst is reduced by coordinating carbon monoxide, hydrocarbons, oxygen, water vapor and the like, the purity of a hydrogen fuel is important. The use of reformed hydrogen containing the above-described substances as impurities is not desired, and as oxygen and hydrogen, pure oxygen and hydrogen raw materials which do not contain CO2 are required.
The chemical reaction formulae in the electrodes of the AFC are as follows.
Whole 2H2+O2→2H2O
Fuel electrode (negative electrode) 2H2+4OH−→4H2O+4e−
Oxygen electrode (positive electrode) O2+2H2O+4e−→4OH
Thus, the AFC has the advantage that as an electrode material, expensive platinum is not required to be used, and a relatively inexpensive metal material such as nickel, cobalt or iron can be used because the electrolyte is alkaline. Meanwhile, if a carbon dioxide gas etc. is mixed in hydrogen that is a raw material, the alkaline electrolytic solution forms a carbonate to be degraded. It is necessary to use high-purity oxygen as an oxidant for achieving a high power.
The present inventors have paid attention to the fact that as a raw material gas, pure hydrogen and oxygen, particularly a raw material that does not contain a carbonaceous substance is required in an alkaline fuel cell (AFC), and found that a hydrogen gas and an oxygen gas which are generated by an alkaline water electrolytic device are most suitable.
Meanwhile, in the alkaline water electrolytic device, a large amount of electric energy is required, and therefore if all the electric energy must be supplied from the outside, enormous costs are necessary.
With attention paid to the above respect, the present inventors have invented a water treatment system using an alkaline water electrolytic device and an alkaline fuel cell in which an alkaline water electrolytic device and an alkaline fuel cell (AFC) are combined with each other, whereby electric power required in the alkaline water electrolytic device and the alkaline fuel cell, a hydrogen gas and an oxygen gas serving as raw materials for the electric power, water for making up for water lost through the electrolytic treatment, and an electrolytic solution formed of the alkaline aqueous solution are effectively used by means of a circulation system within the process, rather than being newly fed from the outside.